The news is based on a review of a large number of studies comparing the health effects of organic foods to conventional foods.

While there is no internationally agreed definition of “organic”, most people understand it to mean:

foods grown without the use of artificial fertilisers, pesticides or other chemicals

meat taken from animals that have not been given antibiotics or growth hormones

Many champions of organic food, such as Prince Charles, have claimed that food grown organically is healthier and more nutritious.

However, this review found no strong evidence to support the health benefits from eating organic instead of conventional foods. This may come as a relief to the more cash-stretched of us because, as the researchers point out, organic food can often be more expensive than conventionally sourced food.

The researchers did find that organic produce was less likely to be contaminated with pesticides. And any bacteria found in organically produced meat were less likely to be resistant to antibiotics.

Obviously there are other reasons, besides nutrition, that may make people choose organic food, such as concern for the environment.

Ultimately, the findings should be interpreted with some caution. There was a high level of variation between the studies in terms of the methods used, which makes the results of this review less reliable. It is also worth noting that few studies looked at relevant health effects and the studies ran for no longer than two years. This means no conclusions about long-term health benefits of organic foods can be drawn from this research.

Where did the story come from?

This review was carried out by researchers from the Stanford School of Medicine and the University of Stanford in California, and other US institutions. It did not receive external funding. The study was published in the peer-reviewed journal Annals of Internal Medicine.

The news stories accurately reflect the findings of this review.

What kind of research was this?

This was a systematic review and meta-analysis that aimed to identify published studies on the health, nutritional and safety characteristics of organic and conventional foods.

A systematic review is considered to be the strongest level of evidence and it is the best way to summarise all of the existing research on the question of interest. During a systematic review researchers should rigorously search for and analyse the best available studies.

These sorts of reviews use set criteria which potential studies must meet to be included, covering appropriate study design, population, intervention or exposure, and outcomes assessed.

The strength of the conclusions drawn from a systematic review are dependent on the quality and homogeneity (sameness) of the studies that it pools together.

As the researchers concede, one of the inherent weaknesses of this study was that organic food production is a broad and complex topic that introduces a wide range of variables.

As a result, its findings may not be as reliable as those of a systematic review or meta-analysis focusing on a narrower issue, such as whether statins can prevent heart disease.

What did the research involve?

The authors searched seven relevant electronic databases to identify published studies that evaluated groups of people consuming diets of foods grown organically compared to conventional foods. There was no restriction on the type of study design and studies were included if they compared nutrient levels or bacterial, fungal or pesticide contamination in the following foods grown organically and conventionally:

fruits and vegetables

grains

meats and poultry

milk

eggs

Studies on processed foods were excluded from the review.

Two independent researchers then assessed the quality of the studies and gathered information including:

methods used in the studies

the amount of organic foods consumed in diets

reported outcomes in the individual studies which were then linked to health outcomes

nutrient levels of the foods

contaminant levels of the foods including pesticides, bacteria, fungal toxins and antibiotic resistance

Researchers then analysed their results using statistical methods and pooled results of eligible studies for the meta-analysis.

What were the basic results?

The researchers identified 237 relevant studies. These included 17 human studies that evaluated health outcomes among groups of people eating organic and conventional foods and 223 non-human studies that compared nutrient and contaminant levels in organic and conventional foods (three of which reported on both human and food outcomes).

The main finding of this study was that organic produce was 30% less likely to be contaminated with pesticides compared with conventional produce (risk difference 30%, confidence interval [CI] -37% to -23%) but that differences in the risk of exceeding the allowed safety limits were small.

In addition there were only three studies out of the 237 (1.26%) where levels of pesticide contamination found in conventional products exceeded EU maximum safety limits.

Another finding was that the risk of bacteria being resistant to three or more antibiotics was higher in conventional pork and chicken compared with organic pork and chicken (risk difference 33%, 95% CI 21% to 45%).

Of the 17 human studies, only three looked at clinical outcomes in terms of the effects on symptoms such as eczema and wheezing. These studies found no significant differences between those eating organic foods compared with conventional foods.

Two studies reported significantly lower urinary pesticide levels among children consuming organic produce but this difference was not observed among adults. It is unclear what, if any, effects increased urinary pesticide levels have on children’s health.

The researchers report that studies were limited in number and varied in their quality. They also report high variation in the studies comparing nutrient and contaminant levels in foods. This was expected and the researchers did not allow results for contamination to be pooled in the meta-analysis.

One of the researchers, Dr Dena Bravata, says, “there isn’t much difference between organic and conventional foods if you’re an adult and making a decision based solely on your health”.

Conclusion

Overall, this systematic review provides some limited evidence of the differences in the health effects of organic and conventional foods. The authors do note that results should be “interpreted with caution” due to the high variation between the included studies. They note these differences may be due to soil type, storage practices and variation within organic practices.

There are some additional limitations to this review. Some of the human studies had very small samples which ranged from six to 6,630 people. In addition, none of these studies ran for longer than two years, which means conclusions about the long-term health benefits of organic foods cannot be drawn. The authors also note that some of the included field studies may not reflect real-world organic practices.

The researchers do suggest that a more effective method of assessing the relative benefits of “organic verses conventional food” would be to carry out a cohort or randomised controlled study. But these types of studies would be both very time consuming and expensive.

Dr Bravata added when discussing the research that, “if you look beyond health effects, there are plenty of other reasons to buy organic instead of conventional”.